ah10Acid-Base balance and disbalance

advertisement

Acid –Base balace and disbalance
The Hydrogen Ion and pH
The hydrogen ion consists of a single positively charged particle (the proton) that is not
orbited by any electrons. The hydrogen ion is, therefore, the smallest ionic particle and is
extremely reactive. It is this fact that accounts for its profound effect on the functioning of
biological systems at very low concentrations.
In the environment hydrogen ion concentrations vary over an enormous scale (from less than
10-14 mol/l to more than 1mol/l).
The pH scale was developed in order to simplify (or
perhaps further complicate!) the mathematics of handling
such a large range of numbers. The pH is calculated by
taking the negative logarithm of the hydrogen ion
concentration, as shown below.
pH = -log10[H+]:
where [H+] is the hydrogen ion concentration.
Figure 1 gives an example of how pH is calculated.
Table 1 gives examples of pH values and corresponding hydrogen ion concentrations. It is
important to note that an increase of one pH point results in a ten-fold decrease in hydrogen
ion concentration.
Table 1: pH and Hydrogen ion concentration
pH
[H+] nanomol/l
6.0
1000
7.0
100
8.0
10
9.0
1
Teaching point
As a solution becomes more acidic or less alkaline, the pH falls (hydrogen ion concentration
rises). The opposite happens when solutions become less acidic or more alkaline
Acids, Bases and Buffers
Acids: An acid is defined as any compound,
which forms hydrogen ions in solution. For
this reason acids are sometimes referred to as
"proton donors". To aid understanding of
these concepts consider an imaginary acid
with the chemical formula HA. In the first
example in Figure 2, the acid dissociates
(separates) into hydrogen ions and the
conjugate base when in solution.
Bases: A base is a compound that combines
with hydrogen ions in solution. Therefore,
bases can be referred to as "proton acceptors".
Strong Acids: A strong acid is a compound
that ionizes completely in solution to form
hydrogen ions and a base. Example 2
illustrates a strong acid in solution, where this
dissociation is complete.
Weak Acids and Bases: these are
compounds that are only partially ionised in
solution. Example 3 shows a weak acid in
solution with incomplete dissociation.
Buffers: A buffer is a compound that limits
the change in hydrogen ion concentration
(and so pH) when hydrogen ions are added or
removed from the solution. It may be useful
to think of the buffer as being like a sponge.
When hydrogen ions are in excess, the sponge
mops up the extra ions. When in short supply
the sponge can be squeezed out to release
more hydrogen ions!
All buffers are in fact weak acids or bases.
Figure 3 shows how as hydrogen ions are
added to a buffer solution they combine with
A- (the conjugate base) and the reaction is
pushed to the left. This creates more HA
whilst removing the excess H+ from the
solution. Similarly, as hydrogen ions are
removed from solution by addition of a strong
base the reaction moves to the right restoring
the hydrogen ion concentration and reducing
the quantity of HA.
The effects of buffers can also be illustrated
graphically. If a strong acid is added slowly to
a buffer solution and the hydrogen ion
concentration [H+] is measured then a plot
similar to the one in figure 4 will be
generated. Notice that during the highlighted
portion of the curve a large volume of acid is
added with little change in [H+] or pH.
As we shall see later buffers are crucial in
maintaining hydrogen ions within a narrow
range concentrations in the body.
The Importance of Hydrogen Ion Concentration
Hydrogen ion concentration has a widespread effect on the function of the body's enzyme
systems. The hydrogen ion is highly reactive and will combine with bases or negatively
charged ions at very low concentrations. Proteins contain many negatively charged and basic
groups within their structure. Thus, a change in pH will alter the degree ionization of a
protein, which may in turn affect its functioning. At more extreme hydrogen ion
concentrations a protein's structure may be completely disrupted (the protein is then said to be
denatured).
Enzymes function optimally over a very narrow range of
hydrogen ion concentrations. For most enzymes this
optimum pH is close to the physiological range for plasma
(pH= 7.35 to 7.45, or [H+]= 35 to 45nmol/l). Figure 5
shows a typical graph obtained when enzyme activity is
plotted against pH. Notice that the curve is a narrow bell
shape centred around physiological pH.
Although most enzymes function optimally around physiological pH it should be noted that a
few enzymes function best at a much higher hydrogen ion concentration (ie: at a lower pH).
The most notable of these enzymes is pepsin, which works best in the acid environment of the
stomach - optimum pH 1.5-3 or [H+]= 3-30 million nanomol/l.
As enzymes have a huge number of functions around the body, an abnormal pH can result in
disturbances in a wide range of body systems. Thus, disturbances in pH may result in
abnormal respiratory and cardiac function, derangements in blood clotting and drug
metabolism, to name but a few. From these few examples it is clear that the anaesthetist
should strive to ensure that hydrogen ion concentration is maintained within the normal
range.
Production of Hydrogen Ions
The processes of metabolism generate hydrogen ions. Small amounts (40-80mmol/24h) are
formed from the oxidation of amino acids and the anaerobic metabolism of glucose to lactic
and pyruvic acid. Far more acid is produced as a result of carbon dioxide (CO2) release from
oxidative (aerobic) metabolism - 15,000mmol/24h (15x103 mmol/24h). Although CO2 does
not contain hydrogen ions it rapidly reacts with water to form carbonic acid (H2CO3), which
further dissociates into hydrogen and bicarbonate ions (HCO3-). This reaction is shown below:
CO2 + H20 <= H2CO3 => HCO3- + H+
This reaction occurs throughout the body and in certain circumstances is speeded up by the
enzyme carbonic anhydrase. Carbonic acid is a weak acid and with bicarbonate, its conjugate
base, forms the most important buffering system in the body.
Acids or bases may also be ingested, however, it is uncommon for these to make a significant
contribution to the body's hydrogen ion concentration, other than in deliberate overdose.
Control of Hydrogen Ion Concentration
With hydrogen ion concentration being so critical to enzyme function, the body has
sophisticated mechanisms for ensuring pH remains in the normal range. Three systems are
involved: blood and tissue buffering, excretion of CO2 by the lungs and the renal excretion of
H+ and regeneration of HCO3-.
1. Buffers
As we have seen, buffers are able to limit changes in hydrogen ion concentration. This
prevents the large quantities of hydrogen ions produced by metabolism resulting in dangerous
changes in blood or tissue pH.
a) Bicarbonate
This is the most important buffer system in the body. Although bicarbonate is not an efficient
buffer at physiological pH its efficiency is improved because CO2 is removed by the lungs
and bicarbonate regenerated by the kidney. There are other buffers that act in a similar way to
bicarbonate, for example: hydrogen phosphate (HPO32-), however, these are present in smaller
concentrations in tissues and plasma.
b) Proteins
As mentioned earlier many proteins, and notably albumin, contain weak acidic and basic
groups within their structure. Therefore, plasma and other proteins form important buffering
systems. Intracellular proteins limit pH changes within cells, whilst the protein matrix of bone
can buffer large amounts of hydrogen ions in patients with chronic acidosis.
c) Haemoglobin
Haemoglobin (Hb) is not only important in the carriage of oxygen to the tissues but also in the
transport of CO2 and in buffering hydrogen ions (The physiology of oxygen delivery, Update
in anaesthesia 1999; 10:8-14).
Haemoglobin binds both CO2 and H+ and so is a powerful buffer. Deoxygenated haemoglobin
has the strongest affinity for both CO2 and H+; thus, its buffering effect is strongest in the
tissues. Little CO2 is produced in red cells and so the CO2 produced by the tissues passes
easily into the cell down a concentration gradient. Carbon dioxide then either combines
directly with haemoglobin or combines with water to form carbonic acid. The CO2 that binds
directly with haemoglobin combines reversibly with terminal amine groups on the
haemoglobin molecule to form carbaminohaemoglobin. In the lungs the CO2 is released and
passes down its concentration gradient into the alveoli.
The buffering of hydrogen ions formed from carbonic
acid is more complicated. The chain of events that occurs
within the red cell is most easily understood by referring
to figure 6.
In the tissues, dissolved CO2 passes into the red blood cell
down its concentration gradient where it combines with
water to form carbonic acid. This reaction is catalysed by
the enzyme carbonic anhydrase. Carbonic acid then
dissociates into bicarbonate and hydrogen ions. The
hydrogen ions bind to reduced haemoglobin to form HHb.
Bicarbonate ions (HCO3-) generated by this process pass
back into the plasma in exchange for chloride ions (Cl-).
This ensures that there is no net loss or gain of negative
ions by the red cell. In the lungs this process is reversed
and hydrogen ions bound to haemoglobin recombine with
bicarbonate to form CO2 which passes into the alveoli. In
addition, reduced Hb is reformed to return to the tissues.
2. Carbon Dioxide Elimination
As mentioned earlier CO2 is responsible for the majority of hydrogen ions produced by
metabolism. Therefore, the respiratory system forms the single most important organ system
involved in the control of hydrogen ions. From respiratory physiology it should be
remembered that the arterial partial pressure of CO2 (PaCO2) is inversely proportional to
alveolar ventilation (ie: if alveolar ventilation falls the PaCO2 rises). Therefore, relatively
small changes in ventilation can have a profound effect on hydrogen ion concentration and
pH. An acute rise in PCO2 of 1Kilopascal (kPa) results in a 5.5nmol/l rise in the hydrogen ion
concentration (resulting in a fall in plasma pH from 7.4 to 7.34).
The importance of PaCO2 and hydrogen ion concentration is underlined by the fact that the
control of ventilation is brought about by the effect of CO2 on cerebrospinal fluid (CSF) pH.
The detail of the control of breathing and elimination of CO2 are beyond the remit of article
but have been discussed in a previous issue of update (Update in anaesthesia 1999; 10:8-14)
3. Renal Handling of Bicarbonate and Hydrogen Ions
The kidneys not only secrete hydrogen ions but they also regenerate bicarbonate ions. The
renal handling of electrolytes also influences acid base balance. All aspects of renal
involvement in acid base balance are interlinked, but for clarity are dealt with separately
below.
a) Regeneration of Bicarbonate:
Bicarbonate ions are freely filtered by the glomerulus. The concentration of bicarbonate in the
tubular fluid is equivalent to that of plasma. If bicarbonate were not reabsorbed the buffering
capacity of the blood would rapidly be depleted.
The process of reabsorption of bicarbonate occurs mostly in the proximal convoluted tubule
and is summarised in figure 7.
Filtered bicarbonate combines with secreted hydrogen
ions forming carbonic acid. Carbonic acid then dissociates
to form CO2 and water. This reaction is catalysed by
carbonic anhydrase, which is present in the brush border
of the renal tubular cells. This CO2 readily crosses into the
tubular cell down a concentration gradient.
Inside the cell the CO2 recombines with water, again
under the influence of carbonic anhydrase, to form
carbonic acid. The carbonic acid further dissociates to
bicarbonate and hydrogen ions. The bicarbonate passes
back into the blood stream whilst the H+ passes back into
the tubular fluid in exchange for sodium. In this way,
virtually all the filtered bicarbonate is reabsorbed in the
healthy individual.
b) Excretion of Hydrogen Ions
Hydrogen ions are actively secreted in the proximal and distal tubules, but the maximum
urinary [H+] is around 0.025mmol/l (pH 4.6). Therefore, in order to excrete the 30-40mmol of
H+ required per day, a urine volume of 1200 litres would have to be produced. However,
buffering of hydrogen ions also occurs in the urine. This allows the excretion of these large
quantities of H+ without requiring such huge urine volumes. Hydrogen ion secretion occurs
against a steep concentration gradient, 40nmol/l in plasma against up to 25000nmol/l
(25x103nmol/l) in urine. Therefore, hydrogen ion secretion is an active process and requires
energy in the form of ATP.
The predominant buffers in the urine are
phosphate (HPO42-) and ammonia (NH3).
Phosphate is freely filtered by the glomerulus
and passes down the tubule where it combines
with H+ to form H2PO4-. Hydrogen ions are
secreted in exchange for sodium ions; the
energy for this exchange comes from the
sodium-potassium ATPase that maintains the
concentration gradient for sodium. These
events are summarised in figure 8.
Ammonia is produced in renal tubular cells by
the action of the enzyme glutaminase on the
amino acid glutamine. This enzyme functions
optimally at a lower (more acidic) than
normal pH. Therefore, more ammonia is
produced during acidosis improving the
buffering capacity of the urine. Ammonia is
unionised and so rapidly crosses into the renal
tubule down its concentration gradient. The
ammonia combines with H+ to form the
ammonium ion, which being ionised does not
pass back into the tubular cell. The
ammonium ion is therefore lost in the urine,
along with the hydrogen ion it contains. See
figure 9 below.
c) Electrolytes
Sodium/Potassium: sodium reabsorption and hydrogen ion excretion are interlinked. Sodium
reabsorption is controlled by the action of aldosterone on ion exchange proteins in the distal
tubule. These ion exchange proteins exchange sodium for hydrogen or potassium ions. Thus,
changes in aldosterone secretion may result in altered acid secretion.
Chloride: The number of positive and negative ions in the plasma must balance at all times.
Aside from the plasma proteins, bicarbonate and chloride are the two most abundant negative
ions (anions) in the plasma. In order to maintain electrical neutrality any change in chloride
must be accompanied by the opposite change in bicarbonate concentration. Therefore, the
chloride concentation may influence acid base balance.
Disorders of Hydrogen Ion Homeostasis
Disturbance of the body's acid base balance results in the plasma containing either too many
hydrogen ions (acidaemia) or too few hydrogen ions (alkalaemia). In other words, the pH is
too low in acidaemia (less than 7.35) whilst in alkalaemia the pH is too high (more than 7.45).
These disturbances may be due to respiratory causes (ie: changes in PaCO2) or nonrespiratory (metabolic) causes. When the cause of the acid base disturbance has been
discovered, the words acidosis or alkalosis may be used in conjunction with the physiological
cause of the disturbance (ie: respiratory acidosis, metabolic alkalosis etc). These are discussed
in more detail below.
Respiratory Acidosis
This results when the PaCO2 is above the upper limit of normal, >6kPa (45mmHg). The
relationship between hydrogen ion concentration and CO2 was discussed earlier (Production
of Hydrogen Ions). Respiratory acidosis is most commonly due to decreased alveolar
ventilation causing decreased excretion of CO2. Less commonly it is due to excessive
production of CO2 by aerobic metabolism.
a) Inadequate CO2 Excretion: the causes of decreased alveolar ventilation are numerous, they
are summarised in Fig 10. Many of the causes of decreased alveolar ventilation are of interest
to theanaesthetist and many are under our control.
b) Excess CO2 Production: respiratory acidosis is rarely caused by excess production of CO2.
This may occur in syndromes such as malignant hyperpyrexia, though a metabolic acidosis
usually predominates. More commonly, modest overproduction of CO2 in the face of
moderately depressed ventilation may result in acidosis. For example, in patients with severe
lung disease a pyrexia or high carbohydrate diet may result in respiratory acidosis.
Respiratory Alkalosis
Results from the excessive excretion of CO2, and occurs when the PaCO2 is less than 4.5kPa
(34mmHg). This is commonly seen in hyperventilation due to anxiety states. In more serious
disease states, such as severe asthma or moderate pulmonary embolism, respiratory alkalosis
may occur. Here hypoxia, due to ventilation perfusion (V/Q) abnormalities, causes
hyperventilation (in the spontaneously breathing patient). As V/Q abnormalities have little
effect on the excretion of CO2 the patients tend to have a low arterial partial pressure of
oxygen (PaO2) and low PaCO2.
Metabolic Acidosis
May result from either an excess of acid or reduced buffering capacity due to a low
concentration of bicarbonate. Excess acid may occur due increased production of organic
acids or, more rarely, ingestion of acidic compounds.
a) Excess H+ Production: this is perhaps the commonest cause of metabolic acidosis and
results from the excessive production of organic acids (usually lactic or pyruvic acid) as a
result of anaerobic metabolism. This may result from local or global tissue hypoxia. Tissue
hypoxia may occur in the following situations:



Reduced arterial oxygen content: for example anaemia or reduced PaO2.
Hypoperfusion: this may be local or global. Any cause of reduced cardiac output may
result in metabolic acidosis (eg: hypovolaemia, cardiogenic shock etc). Similarly, local
hypoperfusion in conditions such as ischaemic bowel or an ischaemic limb may cause
acidosis.
Reduced ability to use oxygen as a substrate. In conditions such as severe sepsis and
cyanide poisoning anaerobic metabolism occurs as a result of mitochondrial
dysfunction.
Another form of metabolic acidosis is diabetic ketoacidosis. Cells are unable to use glucose to
produce energy due to the lack of insulin. Fats form the major source of energy and result in
the production of ketone bodies (aceto- acetate and 3-hydroxybutyrate) from acetyl coenzyme
A. Hydrogen ions are released during the production of ketones resulting in the metabolic
acidosis often observed.
b) Ingestion of Acids: this is an uncommon cause of metabolic acidosis and is usually the
result of poisoning with agents such as ethylene glycol (antifreeze) or ammonium chloride.
c) Inadequate Excretion of +: this results from renal tubular dysfunction and usually occurs
in conjunction with inadequate reabsorption of bicarbonate. Any form of renal failure may
result in metabolic acidosis. There are also specific disorders of renal hydrogen ion excretion
known as the renal tubular acidoses.
Some endocrine disturbance may also result in inadequate H+ excretion e.g.
hypoaldosteronism. Aldosterone regulates sodium reabsorption in the distal renal tubule. As
sodium reabsorption and H+ excretion are linked, a lack of aldosterone (eg: Addison's disease)
tends to result in reduced sodium reabsorption and, therefore, reduced ability to excrete H+
into the tubule resulting in reduced H+ loss. The potassium sparing diuretics may have a
similar effect as they act as aldostrone antagonists.
d) Excessive Loss of Bicarbonate: gastro- intestinal secretions are high in sodium
bicarbonate. The loss of small bowel contents or excessive diarrhoea results in the loss of
large amounts of bicarbonate resulting in metabolic acidosis. This may be seen in such
conditions as Cholera or Crohn's disease.
Acetazolamide, a carbonic anhydrase inhibitor, used in the treatment of acute mountain
sickness and glaucoma, may cause excessive urinary bicarbonate losses. Inhibition of
carbonic anhydrase slows the conversion of carbonic acid to CO2 and water in the renal
tubule. Thus, more carbonic acid is lost in the urine and bicarbonate is not reabsorbed. The
importance of carbonic anhydrase in the reabsorption of bicarbonate was illustrated in Figure
7.
Metabolic Alkalosis
May result from the excessive loss of hydrogen ions, the excessive reabsorption of
bicarbonate or the ingestion of alkalis.
a) Excess H+ loss: gastric secretions contain large quantities of hydrogen ions. Loss of gastric
secretions, therefore, results in a metabolic alkalosis. This occurs in prolonged vomiting for
example, pyloric stenosis or anorexia nervosa.
b) Excessive Reabsorption of Bicarbonate: as discussed earlier bicarbonate and chloride
concentrations are linked. If chloride concentration falls or chloride losses are excessive then
bicarbonate will be reabsorbed to maintain electrical neutrality. Chloride may be lost from the
gastro-intestinal tract, therefore, in prolonged vomiting it is not only the loss of hydrogen ions
that results in the alkalosis but also chloride losses resulting bicarbonate reabsorption.
Chloride losses may also occur in the kidney usually as a result of diuretic drugs. The thiazide
and loop diuretics a common cause of a metabolic alkalosis. These drugs cause increased loss
of chloride in the urine resulting in excessive bicarbonate reabsorption.
c) Ingestion of Alkalis: alkaline antacids when taken in excess may result in mild metabolic
alkalosis. This is an uncommon cause of metabolic alkalosis.
Compensation
From earlier in the article it should be clear that the systems controlling acid base balance are
interlinked. As explained earlier, maintenance of pH as near normal is vital, therefore
dysfunction in one system will result in compensatory changes in the others. The three
mechanisms for compensation mentioned earlier occur at different speeds and remain
effective for different periods.



Rapid chemical buffering: this occurs almost instantly but buffers are rapidly
exhausted, requiring the elimination of hydrogen ions to remain effective.
Respiratory compensation: the respiratory centre in the brainstem responds rapidly to
changes in CSF pH. Thus, a change in plasma pH or PaCO2 results in a change in
ventilation within minutes.
Renal compensation: the kidneys respond to disturbances in acid base balance by
altering the amount of bicarbonate reabsorbed and hydrogen ions excreted. However,
it may take up to 2 days for bicarbonate concentration to reach a new equilibrium.
These compensatory mechanisms are efficient and often return the plasma pH to near normal.
However, it is uncommon for complete compensation to occur and over compensation does
not occur.
Interpretation of Acid Base Disturbances in Blood Gas
Results
Blood gas analysis is available in the vast majority of acute hospitals in the developed world.
Increasingly blood gas machines are available for use in developing countries. In order to
obtain meaningful results from any test it is important that they are interpreted in the light of
the patient's condition. This requires knowledge of the patient's history and examination
findings.
The simplest blood gas machines measure the pH, PCO2 and PO2 of the sample. More
complicated machines will also measure electrolytes and haemoglobin concentration. Most
blood gas machines also give a reading for the base excess and/or standard bicarbonate. These
values are used to assess the metabolic component of an acid base disturbance and are
calculated from the measured values outlined above. They are of particular use when the
cause of the acid base disturbance has both metabolic and respiratory components.


The Base Excess: is defined as the amount of acid (in mmol) required to restore 1
litre of blood to its normal pH, at a PCO2 of 5.3kPa (40mmHg). During the
calculation any change in pH due to the PCO2 of the sample is eliminated, therefore,
the base excess reflects only the metabolic component of any disturbance of acid base
balance. If there is a metabolic alkalosis then acid would have to be added to return the
blood pH to normal, therefore, the base excess will be positive. However, if there is a
metabolic acidosis, acid would need to be subtracted to return blood pH to normal,
therefore, the base excess is negative.
The Standard Bicarbonate: this is similar to the base excess. It is defined as the
calculated bicarbonate concentration of the sample corrected to a PCO2 of
5.3kPa (40mmHg). Again abnormal values for the standard bicarbonate are only due
the metabolic component of an acid base disturbance. A raised standard bicarbonate
concentration indicates a metabolic alkalosis whilst a low value indicates a metabolic
acidosis.
The flow chart on the next page indicates how to approach the interpretation of acid base
disturbances. First examine the pH; as discussed earlier a high pH indicates alkalaemia, whilst
a low pH acidaemia. Next look at the PCO2 and decide whether it accounts for the change in
pH. If the PCO2 does account for the pH then the disturbance is a primary respiratory acid
base disturbance. Now look at the base excess or standard bicarbonate) to assess any
metabolic component of the disturbance. Finally, one needs to decide if any compensation for
the acid base disturbance has happened. Compensation has occurred if there is a change in the
PCO2 or base excess in the opposite direction from that which would be expected from the
pH. For example in respiratory compensation for a metabolic acidosis the PCO2 will be low.
A low PCO2 alone causes an alkalaemia (high pH). The body is therefore using this
mechanism to try to bring the low pH caused by the metabolic acidosis back towards normal.
By now the complexity of acid base disturbance should be clear!! As in many complex
concepts examples may clarify matters. In the following examples work through the flow
charts to interpret the data.
Example 1: A 70 year old man is admitted to the intensive car unit with acute pancreatitis. He
is hypotensive, hypoxic and in acute renal failure. He has a respiratory rate of 50 breaths per
minute. The following blood gas results are obtained:
pH 7.1
PCO2 3.0kPa (22mmHg)
BE -21.0mmol
From the flow charts: firstly, he has a severe acidaemia (pH 7.1). The PCO2 is low, which
does not account for the change in pH (a PCO2 of 3.0 would tend to cause alkalaemia).
Therefore, this cannot be a primary respiratory acidosis. The base excess of -21 confirms the
diagnosis of a severe metabolic acidosis. The low PCO2 indicates that there is a degree of
respiratory compensation due to hyperventilation. These results were to be expected given the
history.
Example 2: A 6 week old male child is admitted with a few days history of projectile
vomiting. The following blood gases are obtained:
pH 7.50
PCO2 6.5kPa (48mmHg)
BE +11.0mmol
The history points to pyloric stenosis. There is an alkalaemia, which is not explained by the
PCO2. The positive base excess confirms the metabolic alkalosis. The raised PCO2 indicates
that there is some respiratory compensation.
Summary
This article has attempted to provide the reader with a
grounding in acid base physiology and its importance in
anaesthesia and intensive care. It is also hoped that the
reader will be able to apply this knowledge in the
interpretation of blood gas results. There is an excellent
web page to learn more about blood gas results
interpretation:http://www.health.adelaide.edu.au/paedanaes/javaman/Respiratory/a-b/AcidBase.html